The last decade has seen a significant amount of research in multi-hop wireless networks (MWNs). While their completely decentralized nature has contributed to scalable solutions, they have also faced significant challenges in moving towards commercial adoption. However, with the next generation wireless networks moving towards smaller (micro, pico) cells for providing higher data rates, there is a revived interest in MWNs from the perspective of integrating them with infrastructure wireless networks. With a decrease in cell size, relays are now needed to provide extended coverage, resulting in a multi-hop network.
User and channel diversity gains, available in conventional one-hop cellular networks, have been effectively leveraged to improve system performance through several channel-dependent scheduling schemes. However, they do not provide spatial reuse. MWNs on the other hand, provide spatial reuse although their large scale makes it difficult to provide hard performance guarantees. Further, since diversity gains require channel state feedback from relay stations and mobile users and must be exploited at fine time scales (order of frames), they cannot be effectively leveraged in a large multi-hop setting. Relay-enabled networks with a two-hop structure, provide a unique middle-ground between these two networks, providing us access to a multitude of diversity and spatial reuse gains. While this provides potential for significant performance improvement, it also calls for more sophisticated, tailored scheduling solutions that take into account the two-hop nature of the system.
Relay-enabled wireless networks have been used to provide improved coverage and capacity. These networks have gained attention both in the standards community (IEEE 802.16j) and in the industry. Scheduling, being a key component in these networks, has received higher emphasis. These systems are typically the TDMA variants where the scheduling decision reduces mainly to deciding whether to employ a relay or not and for which particular user. They focus on link level performance and do not exploit (i) multiple OFDM channels, and (ii) spatial reuse and diversity across the relay and access hops that is available at a network level.
Scheduling forms a crucial component in the efficient exploitation of the spatial reuse and diversity gains delivered by the two-hop relay-assisted cellular networks. Conventional systems leverage only the multi-user and channel diversity gains available in one hop cellular networks. On the other hand, conventional systems that consider scheduling in relay networks do not leverage spatial reuse across hops. Further, conventional relay scheduling solutions do not incorporate finite data in user buffers and instead assume backlogged data. In fact, data in user buffers is limited in practice and incorporation of this aspect along with spatial reuse changes the problem completely, making it more difficult.